Newborn infants, especially those born prematurely, are extremely susceptible to disturbances in fluid, electrolyte, and acid-base homeostasis. Immaturity of kidney function is believed to underlie this susceptibility; as the final regulator of renal excretion, the collecting duct function is critical in this respect. The broad aim of the project is to provide physiologic information on the specific transport processes and transporters during cortical collecting duct (CCD) maturation, with the ultimate goal of providing information necessary for designing preventive and therapeutic measures. To study transport at the segmental and cellular level, experiments using animals are necessary. Newborn rabbits will be used for most studies; prenatal studies (on intracellular pH) will be performed on rat embryonic kidneys. The studies are grouped into 4 broad categories: I. Maturation of Na and K transport in CCD, focusing on transepithelial active and passive transport pathways, development of cell polarity, and activity of NaKATPase. II. Modifiers of Na and K transport maturation in CCD. Potential maturation-enhancing effects of birth, hormones, and growth factors will be assessed. III. Maturation of water transport in CCD. Signal transduction mechanisms underlying the impaired response to antidiuretic hormone will be examined; the maturity of the receptor- adenylate cyclase-protein kinase A pathway, as well as activity of counterregulatory pathways will be studied. IV. Acid-base transport maturation in CCD. A combination of transepithelial transport studies and studies examining intracellular pH regulation will be used to assess the efficiency of transport, as well as development and maturation of specific acid-base transporters. Protocols I.-III. will be performed on neonatal and perinatal rabbit CCD; Protocol IV will, in addition, utilize metanephroi from 13-day-old rat embryos. The methods will include in vitro perfusion of isolated tubules, intracellular pH determination by fluorescent probes, enzyme microassays in isolated tubules, primary cell cultures originating from single isolated tubules, and immunocytochemical methods.